Extractive distillation with furfural



V. SCARTH EXTRAC-TVE DI S T vILLAT ION WI TH FURFURAL July 1s, 1946.

Ill

In Ill 2 dlLS IMENTCR Y vscARTH ATroRNl-:Ys '7 vPatented July 16, 1946 EXTRACTIVE DISTILLATION FURFURAL WITH Virgil Scarth, Bartlesville, Okla., .Vassgnor t to Phillipsrletroleum Company, a corporation Vof Delaware Application December 21, 1944, Serial No. 569,243

This invention relates to extractive distillation, more particularly to extractive distillation of hydrocarbon streams containing unsaturated hydrocarbons using furfural as the solvent, and still more particularly to extractive distillation for the recovery of aliphatic monoolens and/or aliphatic conjugated diolens especially normal butene and/or butadiene from hydrocarbon streams containing same with furfural as the solvent.

It has become customary to recover unsaturated aliphatic hydrocarbons especially butene and/or butadiene from hydrocarbon streams containing them by extractively distilling the stream in a fractional distillation column or absorber into the top of which there is introduced continuously a stream of furfural, usually containing a few per cent of dissolved Water, in amount l suiicient to dissolve the unsaturated hydrocarbon. The top of the column is refluxed with a portion of condensed overhead vapors in the usual Way and the bottom of the column is provided with the conventional reboiler for the purpose of boiling the rich furfural as it attains the bottomso as to free it from undesired paraffin .and/or olefin. The rich furfural is withdrawn s claims. (o1. 19e-13)y It is not desired to be limited to any theory as to the cause of the furfural becoming corrosive.

prevent carryover of furfural into the overhead Y cluding fractional distillation columns and accessories such as piping,` reboilers, condensers, accumulators, pumps and so forth presents a serious problem for which various remedies have been proposed none of which is entirely satisfactory, so far as I am aware. The cause of the formation of corrosive impurities in the furfural is not known even though the problem has been extensively investigated. Factors which may contribute to the formation include high temperature, presence of Water and presence of unsaturated hydrocarbons. It is believed by some investigators that the furfural selectively absorbs traces of corrosive material from the hydrocarbon feed or of material from the feed which be- ;comes corrosive upon reacting with the furfural.

furfural in the re-running operation. other advantageszof such a method will appearhereinafter. w Y

The principal object of the present invention `is to provide` an improved method of conducting It is conventional practice to purify the furfural continuously by re-running a side stream of the furfural leaving the stripper and recycled to the absorber. This is carried out in accordance Vwith U.v S. Patents to K. H. Hachmuth 2,372,668 and 2,350,609, and Buell et al. 2,350,584. However, there is considerable additional loss of furfural during the re-running operation. A large portion of this loss is due to polymerization of the furfural in the re-run unit. At present the furfural is re-run for the purpose of removing otherfcorrosive material as Well as the polymer.; If a feasible method of preventing the accumulation of corrosive material in the furfural in the main system (i. e., in the absorbing and stripping operations) were available, the amount of furfural passed to the re-run unit could be materially decreased with many advantages principal of which is the reduction of polymerization of Many duces the amount of furfural which has to bel passed to the re-run unit. Another object is to prevent corrosion of the main system (i. e., the extractive distillation system proper as distinguished frornthe re-run unit). Another object is to reduce the loss of furfural in the extractive distillation system and in the re-running operation. Another object is to prevent the formation ofcorrosive impurities in the furfural at the point where those impurities are most deleterious. Another object is to accomplish the foregoing objects ina manner which does not cause foaming of the furfural. Numerous other objects will hereinafter appear.

The accompanying drawing portrays diagrammatically one arrangement of equipment forcarrying out the present invention.

'3 V casta I have discovered that the extractive distil- Y lation of hydrocarbon streams with furfural as the solvent to recover unsaturated hydrocarbons therefrom the furfural may be prevented from becoming corrosive and from corroding the equip- Vment by,` intimately contacting the furfural in the extractive distillation system with lime. The term lime as used herein denotes calcium hydroxide. I have determined that limestone will'.` `i

not operate in the process of the present .invention.

The contacting of the furfural with is commonly accomplished `by-introducing the lime in finelyfdivided form into the'furfuralin any suitable manner. The lime may be added n the dry form, as a dispersion or slurry in Vfur-V the inn' I present invention is applicable to the furfural extractivedistillation system in step II or in step IV or in both Or it may be applied in those `butadiene plants where normal butylene feed is catalyticall'y-dehydrogenated-to butadiene and butadiene-is recovered from the eiiluent by extracfural, as a dispersion or slurry in water orbyv other methods. Probably the most convenient method is to add it in the form of a suspension or dispersion in a'portionor all of the makeup water whichv is lcommonly added continuously to the furfural fed to the absorber or absorbersiin order to impart the desired content of dissolved water to the furfural. It will be understood that v addition of"makeup water is for the purpose of replacing water volatilize'd from the furfural in the absorbers and strippers. vThe use of furfural containing dissolved Water as a solvent in the extractive distillation of normal butene and/erbutadiene is disclosed and claimed in the copending application of K. H. Ha'chmuth, Ser. No. 438,- 844, filed April 13, 1942. LA

The present invention is applied t0 the extractive distillation with furfural.

Insu'ch an extractiveV distillation process the i l iurfural enriched in unsaturated hydrocarbon is withdrawn from the bottom of the absorber and passed to the stripper where the unsaturated hydrocarbon is stripped therefrom. The stripped iurfural is then cooled and after addition of makee up water is recycled to the absorber.V Usually a surge tank is provided inthe recycle line. A sidestream of the stripped furfural is continuously fed to a furfural re-run unit wherein it is steam distilled, e. g. in the manner of Buell et al. 2,350,-

, 584, to give a puried material consisting essentive distillation of unsaturated' hydrocarbons `with iurural Wherever the furfural becomes corrosive due to conditions prevailing in the extractive distillation system. The factors which cause the furfural to vbecome corrosive are but little understood but it is believed that they include the high temperatures especially in the reboilingsections of' the absorber and stripper, the presence ofwater andunsaturatd hydrocarbons especially olens and dioleiins, the presence of air or oxygen. The

development of corrosivity in the furfural is aclco'n'ipani'ed bythe formation of polymer which represents alossV ci furfurallandcauses poor heat transfer and other troubles. The presentv invention-is especially applicable to the eXtractive-distillation of butadiene and/or normal Abutene with fur-tural, this 'being the -principal if notrthe `only commercial application of eXtractive distillation with furfural as ameans of lrecovering unsaturated. hydrocarbons. However,l vit may also be applied where furiural is used in the recovery by extractive distillation of otherl aliphatic olefins or aliphatic conjugated diolelns from hydrocarbon streams containing the same.

The conversion of normalk butane to butadiene is now commonly effected by a series of stepsincluding (I) catalytic dehydrogenation of normal butane to normal butene, (II) recovery of normal butene and recycle normal butane by rst fractional-ly distilling the C4 content ofthe .effluent from-step I in a butene-l column wherein butene- 1 is recovered overhead followed by extractive distillation of the resulting bottoms fraction of-n butane and butene-2 withfurfural to separate Vthe butene-Z from the normal butane which is recycled to step I, (III) passing the combined butene-1 and butene-Z recovered in step II to a second catalytic stage and therein dehydrogenating Ysame to butadiene, and iinally (IV) recovering butadiene from the C4V content of the efliuent of tially of furfural and dissolved water and free irornk polymer and other material relatively nonvolatile Awith steam. The re-run furfural is then merge'd with the main recycle stream for passage to furfural storage and ultimatelyto the absorber. 'I'he volume of furfural passed to the re-run unit is preferably such as to maintain the concentration of polymer in the furfural in the system at below 1% by weight.

In accordance with the present invention lim is admixed with the furfural in the extractive distillation system, for example, by suspending. it therein inA any suitable way. It ispreferredgto limit: the amount of lime to that'required to keep` the acidity at the desired level. 'I'he amount of lime required is so small that in practice no difficulty due to sedimentation of the .lime or its reaction products is encountered.Y The lime or a portion thereof may dissolve in the furfural; the presence of dissolved water in the furfural may enhance any such tendency. o

A very convenient method of contacting the furfural with the lime is to employ an excess of a dispersion of lim'efin water,admix it very intimately with the furfural Yinany suitable way,

for example, by means of a ycentrifugal mixing` pump, run the resulting churned-up mixture'or emulsion into a settling tank where it is allowed to separate into two layers, an upper layer of the Water and lime which have. not dissolved in the furfural and a, lower layer of furfural which is saturated with water and which may contain Step III bv a ,Sequence 'Ofsisps .includnsddepropanizing, debutanizing, extractive distillation dissolved lime. 'Ihese layers are separately withdrawn, the water layer being recycled to the mixing lzone to save its water, lime and dissolved furfural content and the furfural layerbeing fed'to the absorber. Under conditions where corrosivevness of thefurfural in the system can beikept .or storage tank for a. period of'time beforeV being re-introduced to the absorber.

of the debutanizer overhead with' furfural wherey aap/i353 the furfural to. reduce the acid content of the v furfural in theextractive distillation system below 0.06 per cent by weight, measured as acetic acid, and still more'upreferably below 0.04 per cent by weight. I have found that below 0.04 per cent of acidity no appreciable corrosion occurs; between 0.04 and 0.06 perv cent acidity the concentration of the corrosive substance whatever it isvmay or may not be suchthat serious-corrosion occurs, apparently depending uponiactors not understood at present; while, at an acidity above 0.06 per cent corrosion. always occurs and, is

.usually so high that the equipment, especially the fractionating columns (i. e. the absorbers) and the strippers, would soon be eaten away below the point of safe operation if the plant continued to'operate at such levels ofv acidity. Furthermore, the loss of furfural by polymerization at levels of acidity above 0.06 per cent becomes excessive so that furfuralY consumption exceeeds the available supply vwhich is exceedingly dangerous to the synthetic rubber program.- The relationship between level of acidity and polymerization of furfural is at. present not understood and I do not wish to be limited to any theory with regard thereto.

. It is preferred to introduce the lime to the furfural at a point in the system such that the fur,- iural is at a temperature of not over 130 F. at the point where the admixture takes place. Usually the coldest pointin the furfural system is just after the cooler or coolers which are employed to cool the furfural leaving the stripper or strippersand so it is preferred to add the lime at this point. The reason for adding the lime at a point of `low temperature` is that the possibility of additional polymerization of furfural upon the introduction of lime to hot furfural is thereby eliminated.

The purpose of lime addition is to reduce the acid content of the furfural, reduce the formation of polymer and the corrosiveness of the furfural. The manner in which vthese several factors are inter-related is not understood but it has been denitely established that the Vaddition of limedoes prevent the furfural from being cor-v rosive and does` reduce the polymer formation,

`whether one causes the other or not. The presentv invention may be employed kfrom th'estart of operations tokeep the furfural in the system from ever becoming corrosive or may be applied to systems in which the furfural hasbecome Vcor- .rosive to eliminate lthe corrosivity and keep the -system thenceforth non-corrosive. ,process of the present invention prevents nonycorrosive furfural from becoming corrosive and also prevents corrosive furfural from exerting its deleterious action. The addition of lime inaccordance with the present invention involves `more than simple neutralization of corrosive acidity since it inhibits polymer yformation as well. Moreover, it has been found by experimental work that neither the acid content nor the pH Aof furfuralis necessarily an index of its corrosiveness. It will be obvious that acid content and VpH are not the same since it is common to have 1a high acid content associated with' a pH ranging from 0 to '7 or a lowacid content anda pI-I lranging fromllto .7.;

Thus the rious corrosion may or` may not -occur and at levels` of acidity above-0.06 percent .the corrosion wliich'occurs is not always serious although it usually is. In any event, whatever the causes of corrosion andregardless of theory it has been proven absolutely that by addition of lime to the furiural in the system in an amount such as to reduce the acidity to, or maintain the acidity at, levels below 0.04 per cent serious corrosion is prevented and polymerization is inhibited; while re,- duction of acidity to, or maintenance atylevels between 0.04 and 0.06 per centiby the addition of lime in most cases prevents seriouscorrosion and reduced polymerization of furiural to satisfactory levels. s

In accordance with the present invention the acid content may be reduced nearly to or to the Aneutral point or the furfural may be made valkaline although the latter practice is not ordinarily followed because the tendency of -furiural to polymerize is enhanced under y alkaline conditions. It is preferred to add only enough lime to reduce the acid content below 0.06 per cent and preferably below 0.04 per cent which practically eliminates corrosion of carbon steel by the lfurfural, however, the nearer the neutral point or the point of zero acidity theless polymers of furfural will be formed. When the acid content of furfural in the main system is below 0.04 per cent the corrosion of carbon steel equipment is negligible and the rate of formation oi polymer is substantially lower than when the acid content is above 0.06

.per cent.

The products formed from the reaction of lime are removed from the system in there-run unit with the polymers, so no problem due to accumulation of lime or its reaction products is encountered. In ordinary operation the re-running is carried out at such a rate that the furfural in the entire system is re-run in a short timesay every two to` three days. If enough lime is added to cause a sediment to form in the main system it may be necessary to provide a trap or filter to remove the sediment from the furfural. vention may be applied by adding lime to asmall side stream of furiural andv any sedimentthat might beformed may be removed by iiltering or some other method before the furfural is returned to the main stream. However, it is voften desirable to add only enough lime to reducethe acid content of the furfural to the extentuindicated but not enough to result in difficulties due to lime or the reaction products of lime settling in the system,

In accordance with my invention the fresh furfural which is added as makeup to the system may be treated with lime prior to its introduction into the extractive distillation system.

In accordance with the present invention lime is added in the main system, i. e. in the extractive distillation system proper, as distinguished from the re-run unit. 'Ihe addition of lime to the y re-run unit is disclosed and claimed in my copending application Ser. No. 607,212, filed July` 26, 1945. s

My in- 7 it iis-often -desirabie-to-appiy the presentimentionf-to extractive distillation` system-s wherein -furural has'rbeoome excessively corrosive. In

suchcaseit may be desirable toadd-enough fur--y furalin' the mainsystem to `reducethe acidity V0.02

. per-cent each day until it is below 0.04 perV cent and -to then reduce the amount of liifneaddeol toa lgure which willhold-theacidityat `thisleveL` Referring to the accompanying drawing, I have illustrated diagrammatically., .the Y `'application of my invention toarplant in .whichbutadiene is made .by the twoesta'ge .catalytic dehydregenation of normalbutane. The mixture of normal butane and butene-Z recovered .from .step I'oiV the .process enters absorber 2 via line I., while the C4 hydro'- carbons uderivedirom step III and containing butene-Z and. butadiene enter absorber .4 via line Y tion.

withdrawn via line '23 and 'fed to refrun unit 24. Vlhe total volume of'furfural in the entiresys tem was 225,000 gallons. Theacidity-ofthe fur*v f'ural in the main system was 0.510 weight per centV product via line and fed to ,stripper 9 where j the dissolved butene-2 is driven 01T. Stripper 9 Vis likewise providedwithy conventional reboiler yandmeans for condensing and returning a portion yof the overhead as reux. v

The undissolved C4 hydrocarbons leave absorber 4 via line I0 While the furfurall rich in butene-2 and butadiene passes via line II lto stripper I2 Whichperformsthe same function asstripper 9, the butene-2 Vand butadiene mixture leaving via line I3. The stripped furfural streamer-leaving stripper 9 via line I4 and stripper I2 viafline I5, are merged and fed via lline IB to cooler Il. YIt will be understood that a cooler may be provided for each stripped furfural stream if desired. The cooled furfural owing vin line I8 is passed via lines I9 and 20 to furfural surge or storage 2| whence it is recycled via line V22 and lines?) and .G to thevabsorbers. A side stream ofrurfural is passedrvia line`23 to the re-run unit 2l whereit Yis continuously pur'ied, inthe manner new well know-n in the art, to removerthe polymer there- Vfrom.V The re-run furfural is `passed via line `2'5 into the main recycle stream inline I9. In accordance with thepresent invention'lime is continuously fed in the properamounts indicated above vinto the main recycle stream Vflowing in line I9. This is accomplished by means of line 26 which, it will be understocd'is `purely .diagrammatic. Usually both lime and water are injected into the stream in line I9. As explained Vabove it is'often convenient to inject the makeup water at this point. A 'point of injection off lime `close to cooler Il is preferred because the temperature is at substantially kthe 'lowest level at this point and because this gives ajre'latively long line between the point of injection of lime Vand furfural surge 2|. Intimate'mixing of the lime with fur'fural may take place in the long pipe indicated by lines I9 and 20. V y f It will be understood that the showing in the drawing is purely diagrammatic and that the lime and water may be added "separately or in any desired manner and that provision maybe made 'for the removal -of any excess lime'or sediment.

It mayor may not benecessary' to-provide for' the removal of the reaction produtsof lime andth'e I impurities in the furfural, depending' uporiltli'e V."II)

per cent by weight.

amount of lime ademend-.the .rate fat :which rur-l In themanufacture ofibuta'diene by 'twoestage 'dehydrogenation of normal butano. afrecoverv system, as lshown in the drawing, was employed.

Furfural was fed continuously into the absorbers 2 and 4 at the rate of '260,000 gai/hr. entering both absorbers. The furfural v'corita'ined-ffoi Water by Wei'ghtrbased on 'the Weight Aof furfnural and water. The stripped furfural from Yboth strippers was cooled to 1Z0-130 F. and the major part' of the cooled product waspassed tothe surge tank 2l. A side'stream of 3500 gah/hr. 'was measured as acetic acid.y This vwasexceedingly corrosive to the equipment and polymer formation was excessive. The acidity had been 'approximately 0.10% lfor ten days, during which time the polymer content increased from 0.4 t'o -1.5

tration and 3500 gal/hr.y of 'iurfural 're-run, equilibrium 'had been established. However., operations were unsatisfactory and it was considered imperative to vreduce corrosionV andi polymerization because otherwise the plant would be corroded away in a short time and it would be impossible to obtain sumcient furfural to continue operations. Y f' Under the foregoing conditions, 'lime wasadded v to the furfural-in the vmainsystem at a rate vof 11 lbs/hr.V 'Ihe lime wasI added in the form of a slurry in the makeup water Whichwas employed to keep the water concentration at approximately 6%. The slurry was admixed with the furfural recycle stream immediately after the cooler.- as vshovvn in the drawing. The acidityY immediately began to decrease at the rate of approximately 0.015% pel' day. When` the acidity reached 0.04%`

the rate of addition of lime was decreased `to 6 lbs/hr. Aand the acidity varied from 0.03Y tol 0.030% from then lon 'for an iridenite-fpe'riod.

The polymer content had'decreased 'to0.3% and remained at abouttiiis figure. Vsimultaneo'usly with the attainment of 0.04% 'acidity,`the'rate 'of reerunning 'of lfurfu'ral was rreduced 'to 2000 gaL/hr. whichfsumced to keep 'the polymer concentration from increasing above 0.3%.

As a result of 'the use of lime in the'manner described inr the foregoing example'the .loss of furfural'in the 'plant Was vreduced vfrom 4000 Yto 1500 gaL/day.V

The amountV of lime necessary to 'be added'to the rurfurai in accordance with the present 1invention 'in order to keep the furfural from becoming corrosive and .the polymer content thereof from increasing is extremely small. Underordinary conditionsoi operation; 'it `ranges from 0.000005 to 'O-.OOOOGlbs/gal. of kf'urfural ied into the yabsorber perhour. Thus,A the total ypounds etlime in the systemat'any onetime isvery small,

'usually less 'thani10'00 lb'sjfor a system `containing 225,000 ygallons furfural as in thefexample.

From the foregoing, many advantages of the present invention will be apparent to those VVskilled in the The'principaliadvantage'is`Y that corrosion of the lequipment'inthe main Y y system is prevented.' This allows the use of equipment of carbon steel'which ismuch' flessrexpensive than corrosion-res'istartmetalarnn- Under these conditions, namely at 0.10% acidity, [1,5% polymer( concene Y other major advantage of the present invention is that it inhibits polymer formation in the main system which of itself gives rise to many advantages well known to those skilled in the art. The invention accomplishes simultaneous reduction in the level of acid concentration in furfural in the extractive distillation system and reduction in the rate of polymerization of furfural. The invention is simple and can be carried out without requiring expensive additional equipment or introducing complications into the existing extractive distillation system and re-run unit. By using lime, there is no danger of high concentration of alkaline material at any point which would be harmful because it would cause an unduly high rate of polymerization of furfural; lime is unable to give a highly alkaline condition by reason of its nature and limited solubility in furfural. The invention is extremely economical to apply because of the small amount of lime and because lime is cheap. The invention greatly reduces the amount of furfural which has to be re-run in order to remove polymer. This reduction in amount of furfural re-run gives rise to many other advantages, the principal of which is the markedly reduced loss of furfural and the lowered cost of re-running the smaller amount of furfural. It will be understood by those skilled in the art that loss of furfural in the re-run system is high because the re-running itself causes additional polymerization and because it is necessary to leave aconsiderable amount of furfural in the polymer in order to get it out of the re-run unit.

The invention is further advantageous in that it accomplishes prevention of formation of impurities at the very point where such impurities are most deleterious, namely in the extractive distillation system which embodies very expensive absorber and stripper units. vantage of the present invention is that it causes no foaming of the furfural in the extractive distillation system. Foaming of furfural in the absorbers and strippers is most objectionable. The behavior of lime introduced in accordance with the present invention is to be compared with that of soda ash, sodium bircarbonate or sodium hydroxide. Undoubtedly such sodium compounds would, if added to the furfural in the main system, substantially decrease corrosion and formation of polymers; however, such alkaline-reacting alkali metal compounds cause ythe furfural to foam which cannot be tolerated in processes of absorption and stripping. Even traces of sodium hydroxide, carbonate or bicarbonate cause foaming of furfural so that it would be completely impossible to use such materials in place of lime in accordance with the present invention.

Another advantage of the present invention is that the amount of lime required is so small that A very marked adno problem due to sediment in the main system is presented. 'I'he lime or its reaction products is completely removed from the furfural in the re-running operation. Many other advantages of the process of the present invention will be apparent to those skilled in the art.

I claim:

1. In the process of separating unsaturated aliphatic hydrocarbons from a mixed hydrocarbon stream containing such unsaturates by extractive distillation wherein said mixed hydrocarbon stream is passed upward through an absorber tower in countercurrent to a descending stream of solvent composed of furfural containing dissolved water; a bottoms product consisting of furfural rich in unsaturated hydrocarbonV is withdrawn from said absorber tower; said bottom product is introduced into a stripper 'column and there stripped of its unsaturated hydrocarbon content; a bottoms product consisting of stripped furfural is withdrawn from said stripper column and split into tWo Streams; one of such resulting streams is passed to a re-run unit `and. there distilled to give a. purified material consisting essentially of furfural `and water disinto that branch of the stripped furfural stream which bypasses the said re-run unit, at a point between the place where said stripped furfural stream is split and Ithe place where the recycled stripped furfural is fed into said absorber tower; and controlling the amount of lime so introduced so that the acidity of the furfural in the extractive distillation system is maintained between 0.2% and 0.6% by weight, calculated as acetic acid.

2. The process of claim 1 wherein the lime is introduced into the furfural in the form of an aqueous dispersion.

3. The process of claim 1 wherein the introduction of said lime is accomplished by admixing same with said branch of the stripped furfural stream which bypasses the re-run unit While said furfural is at a temperature of not over F.

4. The process of claim 1 wherein said hydrocarbon stream is an aliphatic C4. stream and wherein said unsaturated hydrocarbon is an ali- 

